Wide ultrarelativistic plasma beam -- magnetic barrier collision and astrophysical applications

Abstract

The interaction between a wide ultrarelativistic fully-ionized plasma beam and a magnetic barrier is studied numerically. It is assumed that the plasma beam is initially homogeneous and impacts with the Lorentz factor $\Gamma_0\gg 1$ on the barrier. The magnetic field of the barrier $B_0$ is uniform and transverse to the beam velocity. When the energy densities of the beam and the magnetic field are comparable, $\alpha = 8\pi n_0m_pc^2(\Gamma_0-1)/B^2_0\sim 1$, the process of the beam -- barrier interaction is strongly nonstationary, and the density of reversed protons is modulated in space by a factor of 10 or so. The modulation of reversed protons decreases with decrease of $\alpha$. The beam is found to penetrate deep into the barrier provided that $\alpha > \alpha_{cr}$, where $\alpha_{cr}$ is about 0.4. The speed of such a penetration is subrelativistic and depends on $\alpha$. Strong electric fields are generated near the front of the barrier, and electrons are accelerated in these fields up to the mean energy of protons, i.e. up to $\sim m_pc^2\Gamma_0$. The synchrotron radiation of high-energy electrons from the front vicinity is calculated. Stationary solutions for the beam -- barrier collision are considered. It is shown that such a solution may be only at $\alpha \lesssim 0.2 - 0.5$ depending on the boundary conditions for the electric field in the region of the beam -- barrier interaction. Some astrophysical applications of these results are briefly discussed.